I'd second what everyone else said about it probably affecting your ibs in some way in that it will affect hormones - but also that you can't make this decision on the basis of your ibs. Having a child and being unstable for a period of time are VERY different.
Do you have resources around you that can help you? If you are worried about talking to your parents, please go see a school health counsellor, or is there a Planned Parenthood in your area? There is so much support out there for young women and teens having to go through this I would hate for you to feel alone or scared. And of course remember that everyone the boards is here for you!
But of course, take the test first! I've been in that situation before, and no, it is NOT easy. Luckily, my test came back negative even though I was SURE I was pregnant.